The -n tells the operating system to start a new instance, and the +new is for SlickEdit to understand that it is a new instance. If you just want to open the first instance or activate the existing running instance just use
open -a SlickEditV1600
In the open command, file paths come right after the application name, so if you're just opening files (and don't need any other SlickEdit-specific command line args), you don't need the --args stuff. So you would use
> open -a SlickEditV1600 ./*.sh
or
> open -a SlickEditV1600 file1.sh file2.sh
And the file paths will be interpreted as relative to the Terminal shell's current directory.